1. Field of the Invention
The invention relates to force sensors and more particularly to force sensors employing an electrical coil and a movable core.
2. Description of Related Art
Force sensing devices are commonly used in many applications, particularly in automotive vehicles and a variety of subsystems such as brake, suspension, transmission, speed control and the like. Various force sensor are known. Some respond to pressure moving a wiper arm or the like on a potentiometer. Others use piezoelectric elements which generate an output signal representative of the magnitude of an applied force. Still other sensors use a movable core in a coil assembly which provides an output signal representative of the position of the core relative to the core assembly. A problem in prior art force sensors is to obtain output data or an output profile of the applied force which accurately represents the force applied, including minor force differences. A particular difficulty in force sensing is measuring the high forces exerted in these applications without adding travel or mass to a force pedal assembly. A particular problem is to detect the application of relatively small forces such as may be applied at the beginning of travel of a brake pedal or the like. The sensors preferably are rugged and of small physical dimensions to allow them to be used in small spaces and operating under a variety of conditions.
In typical automotive braking systems, the force applied to the brake pedal is transmitted through a master cylinder or the like to the braking system. It is desirable to obtain a measurement of the force applied to the master cylinder and generate an output signal indicative of that force which may be used in another system, such as an electric braking system for a trailer. Known pressure sensing devices which may be used in conjunction with hydraulic brake systems typically lack accuracy, particularly at the initial travel of a brake pedal or the like, often resulting in lost motion. Such lost motion results in a lack of sensitivity in the initial brake pedal travel distance, where touch sensitivity is most important.
A disadvantage of prior art pressure sensing devices which sense fluid pressure is that because of their location in the system, they necessarily respond relatively late in the braking process introducing undesirable delay in signal initiation and, correspondingly, in system response. Furthermore, the cumulative effect of preload springs, frictional forces and other mechanical elements often require the application of significant pedal forces before these sensors react. A further disadvantage of the pressure sensing devices is that they do not function in the event of hydraulic system failure.
Another problem of prior art sensing devices is that only a single output signal is available. For certain applications, such as an automotive braking system, the output of the force sensing device is used to initiate a variety of functions, such as cruise control release, transmission release, shift interlock and suspension control. These functions are preferably performed at levels of applied force. Cruise control release, for example, should occur in response to slight brake pressure and transmission-lock release should occur in response to substantial brake pressure. In existing systems, a single force sensing output signal is used for all purposes.
U.S. Pat. No. 4,818,036, entitled Braking Power Transmitter, discloses a braking power transmitter for use in a pneumatic braking system to produce control pressure for a brake cylinder. The device consists of a foot pedal acting against the force of a coil spring not directly linked to the braking system. An electronic sensing device produces an output signal. A rod, connected to the pedal and extending through the spring, is provided with a core at the distal end which extends into an electrical coil of a sensing device. The sensing device produces an output signal indicative of the position of the core relative to the coil, representing the force applied to the pedal against the spring. A disadvantage of this prior art arrangement, however, is that it does not measure force applied to a brake system, or the like, which provides a reaction force. A further disadvantage of the prior device is that it uses a coil spring which requires either a long stroke or large diameter to measure high forces, such as may be applied to a brake pedal, over short travel distances.